This invention relates to novel corrosion inhibitors for alkanolamine gas treating systems.
Gases such as natural gas, flue gas, and synthesis gas have been purified by the utilization of aqueous alkanolamine solutions for the absorption of acid gases such as CO.sub.2, H.sub.2 S, and COS contained in the gas stream. Ordinarily, a 5 percent to 30 percent by weight alkanolamine solution (e.g., a monoethanolamine solution), flowing countercurrently to the gas stream in an absorption column, is used to remove the acid gases. The process is a continuous and cyclic one which can be reversed at higher temperatures by desorbing the acid gases from the alkanolamine solution.
When steel parts or components are used in such a system, they are subject to both general and local corrosive attack. This is a particular problem in reboilers and heat exchangers where the steel is exposed to a hot, protonated alkanolamine solution. A heat-transferring metal surface appears to be especially vulnerable. Previous investigations by others have revealed that under certain conditions, corrosive products such as aminoacetic; glycolic, oxalic, and formic acids were formed. The alkanolamine salts of these acids present the possibility of increased attack upon ferrous metals. Furthermore, the carbonate salt of monoethanolamine can be converted to additional products such as N-(2-hydroxyethyl)-ethylenediamine which has been found to increase the corrosiveness of the amine solution towards steel, particularly under heat transfer conditions.
There are various alternatives available in order to decrease corrosion rates, among them (1) the provision of a side-stream reclaimer to remove corrosive degradation products, (2) the employment of more corrosion-resistant materials, (3) greater control of the process conditions, and (4) the inclusion of corrosion inhibitors. From both cost and efficiency standpoints, the last alternative is preferred.
Various corrosion inhibitors have been suggested for inhibiting the corrosion of metals in contact with acid-gas absorbing media. For example:
U.S. Pat. No. 4,071,470 discloses a circulating absorbent medium method for inhibiting the corrosion of metals in contact therewith by introducing into said medium a product derived from the reaction of a monoalkanolamine at from about 21.degree. C. to about 100.degree. C., with sulfur or a sulfide and an oxidizing agent, along with copper or a copper salt, sulfide or oxide, for from 0.1 to about 20 hours, until the resulting mixture is stable when diluted with water;
U.S. Pat. No. 4,096,085 discloses a corrosion inhibited aqueous N-methyldiethanolamine or diethanolamine acid gas treating solution consisting essentially of (1) an amine compound or mixture of amine compounds chosen from a particular class of amine compounds; said compound being present in about 10 to about 2000 parts per million parts treating solution; (2) copper or a copper ion yielding compound in from 0 to 1000 ppm; and (3) sulfur or a sulfur atom yielding compound in from 0 to 1000 ppm;
U.S. Pat. Nos. 4,100,099 and 4,100,100 disclose sour gas conditioning solutions. U.S. Pat. No. 4,100,099 relates to a conditioning solution of a combination of one part by weight of a quaternary pyridinium salt and about 0.01-10 parts of a lower alkylenepolyamine, a corresponding polyalkylenepolyamine, or a mixture thereof wherein the alkylene units contain 2-3 carbon atoms. U.S. Pat. No. 4,100,100 relates to a conditioning solution of a quaternary pyridinium salt and about 0.001-10 parts of a thio compound which is a water-soluble thiocyanate or an organic thioamide, and, in addition to the above, a small but effective amount of cobalt, said coablt present as a dissolved divalent cobalt compound; and
U.S. Pat. No. 4,143,119 discloses corrosion inhibitor compositions for ferrous metal and its alloys for absorbent alkanolamine solutions in contact therewith wherein said compositions consist essentially of (a) a source of copper ion selected from the group consisting of copper metal, copper sulfide, and copper salts; (b) a source of sulfur atoms selected from the group consisting of (1) sulfur or (2) hydrogen sulfide and/or COS; and (c) an oxidizing agent which will produce in solution the sulfur atom and at least some polysulfide.
In addition to the aforementioned art, two corrosion inhibited compositions have been disclosed in U.S. Pat. No. 3,896,044 and U.S. Pat. No. 3,808,140.
U.S. Pat. No. 3,896,044 discloses a corrosion inhibited composition consisting essentially of an aqueous alkanolamine solution and an inhibiting amount of a corrosion inhibitor selected from the class of nitro-substituted aromatic acids and nitro-substituted acid salts.
U.S. Pat. No. 3,808,140 discloses a corrosion inhibited composition consisting essentially of an aqueous alkanolamine solution and an inhibiting amount of a combination of a vanadium compound in the plus five valence state and an antimony compound.
The above patents do not disclose the synergistic combination of this invention, i.e. the synergistic combination of an organic compound selected from the group consisting of nitro-substituted aromatic acids and nitro-substituted acid salts, 1,4-naphthoquinone, and mixtures thereof, and particular vanadium compounds wherein the vanadium therein is in the plus four or plus five valence state. In fact, U.S. Pat. No. 3,808,140 claims that only vanadium compounds in the plus five valence state may be employed as effective corrosion inhibitors and then only when employed with antimony compounds.